No CUDA option in system preferences - is it my driver?
IT IS the driver! The Nvidia driver installed from repositories on Debian is old. I've installed the newest driver by using sgfxi (see manual here). Just shut down the X-server, ran the script without any options (for this graphics card you don't need to enable the compositing option, it is enabled by default), it installed without a glitch. And halleluja! - GPU option appeared in Blender.